Ever since Dotter proposed to use a simple wire spiral as a stent, it has been known to build a stent from a spiral pattern of stenting material. One way to move from a relatively small stent diameter to a relatively large diameter, when the stent is a spiral, is for the number of turns of the spiral to go down, as the diameter goes up. Another way is to build the spiral from a succession of zig-zag struts. In a radially compact (unexpanded) configuration, the struts can all lie more or less parallel with the longitudinal axis of the stent, with the points of inflection at each end of each strut of the zig-zag causing the line of strut material to double-back on itself to extend around an angle of 180° into the respective adjacent struts on either side. A very substantial radial expansion can be achieved by opening up the zig-zags, so that the struts no longer lie parallel to each other and with the long axis of the stent but, rather, in a more open zig-zag, where the angle between two succeeding struts of the zig-zag, at the intervening point of inflection, is more like 120° than the original 180°. One example of such a stent is disclosed in U.S. Pat. No. 6,911,041 B1.
One big advantage of a spiral pattern for a stent is that it is a great deal more flexible in bending than a simple annular cage of struts such as one finds in the early Palmaz stent. However, the price of such great flexibility is that the stenting force available from a spiral stent, to push bodily tissue away from the long axis of the stent, is liable to be inherently significantly less than with a cage stent.
Various spiral pattern stents are known from WO 2007/095466 A2, as well as in the prior art citations acknowledged therein.
It would be desirable to be able to retain the flexibility of a spiral stent yet achieve, at the same, enhanced magnitude radially outwardly directed stenting forces.